The choice of Ryan would seem to confirm two perpetual truths about Gov. Romney and President Obama: the latter is the luckiest politician since Reagan, and the former is the worst presidential candidate since Dukakis. The first premise flows naturally and inexorably from the second.
Once it became stunningly obvious that he was blowing it, Romney had two ways to go in this race. He could a) move more briskly and openly to the center-right, trusting that his base would vote not for him but nonetheless feverishly against Obama, or b) he could nervously shore up his base by selecting as his running mate a certified snake-oil pitchman who would imperil the independent vote.
There was risk involved in choice "a"--risky, because "a" entailed trust. Romney is clinically risk-averse, however, and he Nixonianly drips paranoia and therefore the quality of ever trusting in others. In choice "b," though, greater certainty awaited. Sure there would be independent casualties, but his core strike force would remain unmolested.
And it is here that we must recall the ever-shrinking circumference of far-right GroupThink. Romney has surrounded himself for several years now with narrow ideologues who've been uniquely capable of guiding him through the minefields of farther and farther right-winging fanaticism and madness. Romney may have been born absent any ideological bones, but his advisers are the sturdiest of True Believers--and week by week, month after month, they have sucked Mitt Romney into that delusional pseudoconservative hole which casually dismisses pregnant electoral facts (e.g., vast majorities oppose injury to Medicare) and swallows instead the far-right wonders of vastly overconfident, self-satisfied triumphalism.
In short, they think they're the majority. Hence they've convinced Romney that he needs the base far more than he requires independents.
That, anyway, is my educated guess--and the principal reason why President Obama is the luckiest politician alive.